1. Field of the Invention
The present invention relates to an improvement in the production of anhydride-functionalized polymers useful as adhesives for bonding various resin substrates. More specifically, the invention relates to the modification of polymers grafted with maleic anhydride which are used as adhesives for multi-layer film constructions wherein one or more of the resin layers is an EVOH resin. The improvement consists of reacting the grafted anhydride functionality with an alcohol, up to one equivalent alcohol per equivalent of anhydride, to improve the optical quality of films produced using the resulting product.
2. Description of the Prior Art
Multi-layer film products wherein one or more of the resin layers is a polyolefin and one or more of the remaining resin layers is a homopolymer or copolymer of vinyl alcohol are widely utilized by the packaging industry where moisture and oxygen barrier properties are required. Both of these resins are coextrudable and well-suited for the construction of multi-layer films. The polyolefin in the structure provides a high degree of impermeability to water vapor and resistance to chemicals while the vinyl alcohol resin supplies the deficiency of the polyolefin with regard to the permeability of oxygen and other gases. Vinyl alcohol polymers are well-known for their low gas permeability and ethylene-vinyl alcohol (EVOH) copolymers are particularly useful in view of their ready coextrudability. One or more other resin layers e.g. polyamide, ethylene-vinyl acetate (EVA) copolymer, polyester or polyvinylidene chloride, may also be included in the construction to impart other desirable characteristics to the film.
Since vinyl alcohol polymers do not adhere well to several of the more commonly used resins, most notably polyolefins and EVA copolymers, an interlayer of adhesive material which exhibits good adhesion to both resin substrates is generally coextruded between them. This adhesive material, sometimes referred to as the "tie-layer", makes it possible to obtain a multi-layer film which has high resistance to the passage of gases and moisture and wherein the layers are resistant to delamination. Such multi-layer film products are well adapted for use as packaging materials for both food and non-food products which require durable moisture and oxygen barrier packaging.
Modified polyolefins are extensively utilized as the interlayer material. Most commonly, the modified polyolefin is a polyethylene or polypropylene resin which has been modified by grafting with an unsaturated fused-ring carboxylic acid anhydride, e.g., maleic anhydride. These grafted polymers are typically blended with one or more other polymers to achieve the desired balance of properties. A variety of high performance adhesives for coextrusion and metal bonding which incorporate anhydride-functionalized polyolefins are manufactured by Quantum Chemical Corporation and sold under the PLEXAR trademark.
Even though coextruded films with acceptable levels of adhesion can be produced using the anhydride-functionalized polyolefin blends, in some cases there may be a corresponding decrease in the optical qualities of the resulting multi-layer film structure. In severe cases, the film is hazy or cloudy which is, of course, objectionable to users who desire clear, water-white film to display their packaged goods. While the optical quality of most films can be brought up to acceptable levels by judicious selection and control of process conditions, such as by controlling temperature of polymer melt(s), varying the length of time the resin melts are in contact and the rate of cooling, it is not always possible in commercial operations to vary these parameters to the extent necessary due to equipment limitations and the adverse affect these changes can have on other properties. For example, while decreasing the melt temperature of the polymers can improve the optical quality of the resulting film, it reduces adhesion. Similarly, decreasing the length of time which the molten adhesive blend is in contact with the polymer melt improves optical clarity but lowers adhesion values. Cost and manufacturing considerations must also be taken into account so that even though optical quality and adhesion can be increased by reducing the cooling rate of the film as it exits the extruder, this is generally not an acceptable approach for commercial operations where high line speeds are essential.
For the coextrusion of EVOH or other vinyl alcohol resins it is therefore most desirable to employ adhesive blends which can be used in existing coextrusion operations without significantly changing operating conditions or equipment design. To this end, it would be advantageous if the anhydride-functionalized polymers utilized for these adhesive blends and which are known to adversely affect the optical quality of films formed therewith could be modified to eliminate or substantially reduce the problem. It would be even more advantageous if this could be accomplished without any reduction in adhesion.
It is generally accepted within the industry that the optical quality of a coextruded multi-layer film is not determined by a single property. Rather, optical quality is determined by a balance of several properties, namely gloss, haze and transparency. Gloss is a measure of the shiny appearance of the film surface whereas haze relates to the light-transmitting ability of the film. Transparency is a measure of the ability to "see through" the film without distorting relatively distant objects. Whereas gloss and haze are generally considered to result from the bulk properties of resins used, poor transparency is in many cases believed to result from an interfacial phenomenon sometimes referred to as "interfacial instability." It is possible for films to have good values for two of these properties and yet be unacceptable due to a deficiency of the remaining property. For example, a film may have acceptable gloss and haze and yet be unacceptable to a manufacturer due to its poor transparency. A balance must be struck between these three properties and no one of the properties may fall below its minimum acceptable limit if a film is to be considered truly acceptable.
Since the use of various anhydride-functionalized polymers has been observed to detract from the transparency of coextruded multi-layer EVOH films, it would be highly desirable if anhydride-functionalized polymers which yield significantly improved transparency could be produced. It would be particularly advantageous if these improved polymers did not detract from the adhesive properties or the gloss and haze properties of the film. These and other advantages are realized by the modified anhydride-functionalized polymers produced by the process of the present invention.